The quality of mold casting is affected a great deal by the process of filling the molds with molten metal. It is extremely important to quickly flood the sprue cup of the mold and maintain it full while metal propagates through the gating system of the mold into the mold cavities. This assures high quality castings without voids, misruns and gas entrapments. In addition, to prevent massive spills of molten metal, it is necessary to control the flow of the molten metal stream during the final stage of a pour (usually referred to in the prior art as "cut off") so that so-called "metal in transit" from a casting ladle or other source will just fill the mold without over-filling it.
Traditionally, this pouring operation is performed manually. Prior attempts to automate the process have not been successful. Unsuccessful attempts include tilting a lip-type ladle or opening a bottom pour ladle for a given period of time. (See U.S. Pat. Nos. 3,838,727, 3,842,894 and 4,276,921). However, these methods fall short of that desired because they lead to frequent overpours or underpours since the flow of metal is not controlled.
Another unsuccessful prior art method utilizes optical sensors which detect molten metal rising through vents in the mold. The sensors output a signal which activates a mechanism to cut off the flow. However, this method also has several drawbacks. For one thing, actual flow into the molds is not controlled as a function of the mold. Moreover, the detection of full vents, so called "pop-offs," results in over-filling the mold, since the "mold full" signal is detected too late, and the "metal in transit" to the mold cannot be accommodated by the mold. The method is also unreliable because splashes of molten metal around the sprue cup can confuse the sensor and cause premature flow cut off.
Still another method is described in U.S. Pat. No. 4,304,287. In the method described in that patent, two optical sensors are utilized. One measures the intensity of light emitted by the molten metal stream. The second measures the intensity of light emitted by the molten metal in the sprue cup. The analog outputs from these sensors are compared with reference values and, when rapid change in the signal from the light sensors is detected, indicating that the mold is nearing the full condition, the flow is cut off. This method is better than sensing full "pop-offs", but nevertheless has its disadvantages. The level of molten metal in the sprue cup is referenced to an absolute reference, the position of the sensor, and not to the surface of the mold. The system cannot accomodate for wide variations (typically .+-.1/2 inch) in mold dimensions. Moreover, the flow cut off signal is generated relatively too late and may still result in spill-overs of metal in transit to the mold. In addition, slag in the sprue cup can reduce the intensity of light detected by the sensors and lead to errors in generating the flow cut off signal.
The present invention differs significantly from known methods in that it controls the flow of molten metal into the molds so that the flow rate of metal into the sprue cup is always equal to the flow of metal into the mold through the mold gating system. The flow rate is not necessarily constant. In addition, the method of the present invention can be made adaptive so that the flow of metal is controlled during the entire pour based on information obtained from previous pours, so that the flow rate is controlled while also accomodating the metal stream in transit to the mold, so that the metal in transit just fills the mold rather than overfilling it. Pour parameters can be updated after each pour, enabling the system to "learn" how to fill a new mold precisely after just a few pours.
Moreover, the present invention is not adversely affected by external factors such as changes in the viscosity of molten metal, or changes in the diameter of the metal stream. It is also independent of metal height, or "head", in the pouring ladle or other molten metal reservoir.
The present invention has several additional benefits. It provides the capability for the accurate position of the molten metal stream directly in the center of the sprue cup. And, by analyzing the pour control signal generated by the present invention, it is possible to detect abnormalities caused by a broken mold or other malfunctions in the mold filling system.